This program represents a continuing, broadly based study of several aspects of lymphocyte immunobiology with particular emphasis on the specificity of T lymphocytes, the cellular basis of the immune response mechanism and how properties of lymphocytes become altered in neoplastic states. Specifically, the component projects deal with: 1. Functional, genetic and structural studies of T cell receptors; 2. The origin of diversity in the B cell specificity repertoire; 3. Various aspects of lymphocyte function in bone marrow chimeras; 4. Response to MHC alloantigens in relation to marrow transplantation: 5. The immunobiology of weak transplantation alloantigens; 6. The immunobiology of MHC expression on macrophages; 7. Membrane markers of normal and neoplastic human lymphocytes; 8. Molecular membrane events in lymphocyte mitogenesis; and 9. Regulation of anti-MHC responses with monoclonal alloantibodies. This program is a component of The University of Pennsylvania Cancer Center.